1. Field of the Invention
The present invention relates to a screen display unit for displaying characters such as a channel number on a television screen.
2. Description of Related Art
FIG. 9 is a block diagram showing a configuration of a conventional screen display unit. In this figure, the reference numeral 1 designates a CPU for writing character codes into a display RAM which will be described later, and for writing control data into a register; and 2 designates a data and address bus.
The reference numeral 3 designates a display RAM to which the CPU 1 writes character codes to be displayed at individual positions on a screen consisting of n row by m column character array; and 4 designates a font data memory for prestoring font patterns corresponding to the character codes, and display color codes of individual dots constituting the font patterns, and for reading display color codes constituting the font patterns corresponding to the character codes read from the display RAM 3. The reference numeral 5 designates a color palette for prestoring display color data (RGB allocation information) corresponding to display color codes, and for reading the display color data corresponding to the display color codes read from the font data memory 4; and 6 designates a color palette for prestoring display color data that correspond to the display color codes and differ from the display color data stored in the color palette 5, and for reading the display color data corresponding to the display color codes read from the font data memory 4.
The reference numeral 7 designates a register into which the CPU 1 writes control data; 8 designates a selector for selecting the display color data read from the color palette 5 or 6 in response to the control data written in the register 7; 9 designates DA converters provided for individual RGB signals for converting the digital display color data selected by the selector 8 to analog voltages; and 10 designates an output terminal. The reference numeral 11 designates a timing generator for generating, from the horizontal and vertical synchronization signals and a screen display clock signal, timing signals that are supplied to the components from the display RAM 3 to the DA converter 9, and are needed for screen display control.
Next, the operation of the conventional screen display unit will be described.
In the screen display unit as shown in FIG. 9, the CPU 1 writes a character code array to be displayed into the display RAM 3 via the bus 2 in advance, and writes the control data indicating which one of the color palettes 5 and 6 is to be selected into the register 7 via the bus 2. The operation clock signal of the CPU 1 is asynchronous to the timing signals generated by the timing generator 11 to operate the components from the display RAM 3 to the DA converter 9, and is slower than the timing signals.
The display RAM 3 reads the character codes written in synchronization with the timing signals, and supplies them to the font data memory 4. The font data memory 4 reads the display color codes of the individual dots constituting the font patterns corresponding to the character codes. The color palettes 5 and 6 store the different display color data in advance, and read the display color data corresponding to the display color codes that are read from the font data memory 4.
On the other hand, in response to the control data written into the register 7 by the CPU 1, the selector 8 selects the display color data read from the palette 5 or 6. The DA converters 9 convert the digital display color data selected by the selector 8 into analog voltages to be supplied to the output terminal 10.
Since the conventional screen display unit is configured as describe above, it can select 2n pieces of display color data from the color palette 5 when the display color codes designating the colors of the individual dots consist of n bits in the font data memory 4. Although the number of colors can be increased with the bit length of the display color codes, this will increase the bit length per dot, and hence the capacity of the font data memory 4, and by extension the cost of the screen display unit itself. With the current state of the art, the number of display colors is doubled by installing two color palettes with different display color data such as the color palettes 5 and 6, and by switching the control data of the register 7 on a screen by screen basis (at every vertical synchronization signal interval) by the CPU 1.
The conventional system, however, cannot switch between the color palettes 5 and 6 at a high speed within the same screen interval because the processing cycle of the CPU 1 is slower than and asynchronous to the screen display operation. As a result, the conventional system has a problem of being unable to increase the number of colors displayed on the same screen.